A display panel causes light emission from pixels to display images. The display panel generates heat under the light emission from the pixels. Various radiators have been developed to facilitate heat dissipation from display panels.
Patent Documents 1 and 2 disclose radiators formed from graphite sheets. Each of Patent Documents 1 and 2 attempts to use a graphite sheet covered with another sheet material which prevents graphite powder from dropping off from the graphite sheet.
A graphite sheet has excellent heat conductivity in the in-plane direction. Therefore, the graphite sheet may be used as a suitable material to facilitate thermal dissipation from an area in which there is heat generation.
FIG. 13 is a schematic view showing effects of a radiator on images displayed on a display panel 900 which uses organic EL (electroluminescence) elements (organic EL display panel).
The display panel 900 shown in the section (a) of FIG. 13 displays a locally bright image. The rectangular area at the center of the display panel 900 is a high brightness area 901 in which a high brightness image is displayed whereas the outer area surrounding the high brightness area 901 is a low brightness area 902 in which a low brightness image is displayed. The high brightness area 901 generates a lot of heat.
The section (b) of FIG. 13 shows an image determined by video signals subsequently transmitted to the display panel 900. The video signals determine an evenly bright image. Brightness of the image determined by the video signals is slightly lower than the brightness of the high brightness area 901.
The section (c) of FIG. 13 shows an image actually displayed on the display panel 900 without a radiator. Heat generation in the high brightness area 901 causes a difference in luminous efficiency between organic EL elements situated in the high and low brightness areas 901, 902. Accordingly, an afterglow area 903 substantially in correspondence to the high brightness area 901 appears in the image.
The section (d) of FIG. 13 shows an image actually displayed on the display panel 900 with a radiator. Since the radiator radiates heat generated in the high brightness area 901, the difference in luminous efficiency of the organic EL elements is decreased between the high and low brightness areas 901, 902. Therefore, there may be a decreased area size of the afterimage area 903. Thus, preceding image display becomes influential to the display panel 900. Therefore, it is important to mount a radiator on the display panel and decrease heat generation.
The present inventor also figured out that a display panel has heat sources other than the display area to display images.
FIG. 14 is a schematic plan view of a substrate 910 of a display panel. The heat sources other than the display area are described with reference to FIG. 14.
An external substrate (not shown) is mounted on the substrate 910 to supply power to the substrate 910. In FIG. 14, the external substrate is mounted on the rectangular area MA indicated by the dotted line. An electrode 911 is situated in the rectangular area MA to supply power to the substrate 910.
An electrode 913 is arranged in correspondence to the electrode 911 in the rectangular area MA. The electrode 913 is situated on a power supply zone 914 to supply power to a display area. The electrode 913 is narrower than the electrode 911 in the rectangular area MA. This means that a power line 915 connecting the electrodes 911, 913 tapers toward the electrode 913.
The graph above the plan view of the substrate 910 schematically shows a change in resistance value between the electrodes 911, 913. Because of the power line 915 which gradually becomes narrower, the resistance value reaches a peak at the boundary between the power line 915 and the electrode 913. Accordingly, there is large heat generation at the boundary between the power line 915 and the electrode 913. This heat is transferred to the electrode 913 through the power line 915. There is great contact resistance between the electrode 913 and the power supply zone 914. Because of this contact resistance, heat is also generated in a contact area of the electrode 913 on the power supply zone 914.
FIG. 15 is a schematic plan view of a substrate 920 of a display panel. The heat sources other than the display area are further described with reference to FIG. 15.
An external substrate (not shown) is mounted on the substrate 920 to supply power to the substrate 920. In FIG. 15, the external substrate is mounted on the rectangular area MA indicated by the dotted line. An electrode 921 is arranged in the rectangular area MA to supply power to the substrate 920.
An electrode 923 is arranged in correspondence to the electrode 921 in the rectangular area MA. The electrode 923 is situated on a power supply zone 924 to supply power to a display area. The electrode 921 is narrower in the rectangular area MA than the electrode 923 is. This means that the power line 925, which connects the electrodes 921, 923, tapers toward the electrode 921.
The graph above the plan view of the substrate 920 schematically shows a change in resistance value between the electrodes 921, 923. Because of the power line 925 which gradually becomes narrower, the resistance value reaches a peak at the boundary between the power line 925 and the electrode 921. Accordingly, there is large heat generation at the boundary between the power line 925 and the electrode 921. This heat is transferred to the electrode 923 through the power line 925. Like the configuration in FIG. 14, because of this contact resistance generated between the electrode 923 and the power supply zone 924, there is heat generation in the contact area between the electrode 923 and the power supply zone 924.
If heat generated on the power supply path is transferred to the radiator, the radiator conducts the heat to the display area which displays images. Accordingly, luminous efficiency of luminous elements in the display area fluctuates because of the heat generation on the power supply path.
Patent Document 1: JP 2005-210035 A
Patent Document 2: JP 2008-80672 A